Nearly all currently manufactured electronic products, including devices such as computers, cell phones and other consumer electronics, are constructed by attaching electronic components to substrates. Electronic components include integrated circuits, passive devices, displays and connectors. Substrates function to hold the electronic components in place and provide electrical connections between the components with desired mechanical, thermal and electrical properties. Substrates typically include an electrically non-conductive layer or layers combined with electrically conductive elements that function electrically in cooperation with the electronic components. Materials that form the non-conductive layers can include crystalline materials such as silicon or sapphire, amorphous materials such as non-crystalline silicon or glass, sintered ceramic materials such as aluminum oxide, or organic materials such as FR-4, polyimide, or ABF, or combinations of the preceding materials. Conductors are formed on or in the substrate by processes including photolithographically depositing conductive materials such as polysilicon, aluminum or copper, depositing conductive inks using screen print or ink jet technologies, or laminating and/or patterning conductive layers on or in the substrate.
What these processes have in common is the need to interconnect electrical conductors that may be separated by layers of electrically nonconductive material. Electronic substrates are typically made up of conductive and nonconductive layers arranged in a planar fashion. FIG. 1 shows a schematic diagram of a multilayer substrate made up of electrically conductive or inorganic layers 10, 12 and 14, separated by electrically nonconductive or organic layers 20, which may contain one or more reinforcing layers 24.
The performance of a laser via drilling system is evaluated according to several criteria, including throughput, accuracy and via quality. Factors that determine via quality include location, shape, debris and taper. Taper refers to the shape and angle of the via side walls. Side wall taper is important because, following drilling, vias are typically plated with an electrically conductive material such as copper to electrically connect layers of a multilayer substrate. High taper, where the walls are relatively parallel, allows the plating to be of high quality and durable.
Drilling high quality vias with a specific taper is highly desirable because it makes it easy to provide good electrical and mechanical contact between the conductor at the bottom of the via and the conductor at the top. Furthermore, providing a good, textured surface, free from debris or remaining organic “smear,” enables good electrical contact between the bottom conductor and the plating, further improving the via quality. At the same time, it is desirable to maintain as high a system throughput as possible, meaning that as little time as possible should be taken to drill a via. Given a maximum repetition rate of a laser, this usually means drilling the via with as few pulses as possible, consistent with desired taper and quality. And finally, it is desirable to deliver a system and method to accomplish the above at a reasonable cost and complexity.
U.S. Pat. No. 6,479,788 of Arai, et al., assigned to Hitachi Via Mechanics, Ltd., attempted to solve this problem by varying the pulse width of substantially square pulses as the via is being drilled. The difficulty with this approach is that it requires very precise control of the laser pulses at very high speed. Since today's lasers may exceed 30,000 pulses per second, this system requires control and optics capable of modifying pulses with possible nanosecond resolution at very high power at very high pulse rates, which likely reduces the system reliability and increases cost. It would be desirable then, to achieve the desired taper, quality and throughput without requiring elaborate real time control of each pulse.
There is a continuing need for an apparatus for laser drilling vias in electronic assemblies that is capable of forming vias with high taper, while maintaining acceptable system throughput, accuracy and overall quality.